Miniature micromachined microphones have gained great popularity in a variety of applications. Because of its sub-mm size, low cost for mass production, lower power consumption, higher sensitivity and reliability, it is widely recognized as the next generation product to replace the conventional electrets condenser microphone (ECM) in applications such as hearing aids, cell phones, PDAs, laptops, MP3, digital cameras etc. Among all the micromachined microphones, the capacitive condenser type of microphone has many advantages over other technical approaches such as piezoelectric or magnetic type micromachined microphone for its smaller size and higher sensitivity etc.
The micromachined condenser microphone typically consists of an acoustic pressure sensing element, generally a variable capacitor, and a preamplifier IC circuit. One prior art example of a condenser microphones with a parallel plate capacitor is disclosed in U.S. patent publication no. 2006/0093170 (Zhe et al.) entitled “Backplateless silicon microphone”. The prior art suffers from some or all of shortcomings mentioned below due to the structure and sensing motion of the parallel plate variable capacitor.
First of all, residual film stress on the diaphragm reduces the sensitivity of the microphone. Since the compliant diaphragm is usually made of a thin film of dielectric and electrically conductive materials, it is very difficult to control or reduce its residual stress because the residual stress is present after the film formation. Stress on the diaphragm has a direct impact on the sensitivity of the microphone. Compressive residual stress results in a defective, buckled diaphragm. Tensile stress severely decreases the sensitivity of the microphone, or totally ruptures the diaphragm at the worst cases.
Secondly, stiction between a flexible diaphragm and a rigid backplate can result in either a faulty device during microfabrication or malfunction during operation. When the gap between the compliant diaphragm and the backplate is on the order of several microns, the diaphragm will adhere to the fixed backplate with a larger probability because the surface to volume ratio increases and surface forces, which are responsible for stiction, are correspondingly higher. Stiction could prevent the successful releasing of the suspended compliant diaphragm during the wet process of the sacrificial layer etching, leading to permanent adhesion to the fixed backplate. During the operation, if the microphone is exposed to a humid environment, water vapor can condense and form a water film on the diaphragm and backplate surfaces. When the gap between the two surfaces decreases during operation and the water film of one surface touches the counter surface, the two surfaces will stick together.
Thirdly, “squeeze film” air damping affects the high frequency response, and contributes noise to the microphone output by generating pressure fluctuations in the microphone structure. For the sub-mm-sized capacitive condenser microphone, the air gap must be scaled down to several microns to keep the capacitance value in a range which can drive the input of the buffer amplifier effectively. However, as the air gap is reduced, the “squeeze film” damping effects due to the viscous flow of air trapped between the diaphragm and backplate increases rapidly. “Squeeze film” air damping can also impact the sensitivity of the microphone.
Fourthly, the “pull-in” effect of the diaphragm reduces the DC bias voltage, which therefore lowers the sensitivity of the microphone. A higher DC bias voltage between diaphragm and backplate yields higher sensitivity. A higher DC bias voltage will create a larger attractive electrostatic force between the diaphragm and backplate. However, in some prior art examples, the gap between the diaphragm and backplate is reduced to several microns, and the mechanical compliance of the diaphragm is kept fairly low in order to have some deflection under certain sound pressure level. Larger attractive electrostatic force can overcome the mechanical restoring force of the diaphragm, and can pull the compliant diaphragm over the small gap to touch the backplate. This phenomenon is called the “pull-in” effect.
Fifthly, a sub-mm-sized diaphragm that is fully constrained by the surrounding frame reduces the sensitivity of the microphone. The compliance of the diaphragm tends to decease very rapidly with the decreasing size for a given diaphragm material and thickness. The mechanical compliance/stiffness of the diaphragm for the sound pressure scales as the fourth power of the diaphragm size.
Sixthly, the small air gap and compliant diaphragm of parallel plate type capacitive condenser microphones can't provide a large dynamic range as higher sound pressure levels could drive the flexible diaphragm to contact the backplate across the small air gap.
Seventhly, the parasitic capacitance between the flexible diaphragm and rigid fixed backplate degrades the microphone performance. The capacitance between the diaphragm and backplate has two parts. The first part varies with acoustical signal and is desirable for microphone. The second part is a parasitic capacitance which does not vary with acoustical signal. The parasitic capacitance degrades the performance and should be minimized. However, the parasitic capacitance is related to the construction of the parallel plate type of silicon microphone in the prior arts.
Last but not least, the parallel plate type capacitive condenser microphone is fairly complicated and costly for manufacturing. So far, the prior art has been unable to provide an economic manufacturing method for the mass production of microphones. Some manufacturing methods of sensing elements disclosed in the prior art are not compatible with standard IC CMOS process, resulting in larger hybrid package and higher manufacturing cost.